A ring oscillator is typically comprised of a number of inverters connected in series as a ring. The inherent delays within each inverter causes the ring oscillator to oscillate in proportion to the total gate delay by the inverters in the ring.
If single-ended delay elements are employed, then there must be an odd number of inverters in the ring, as shown by the ring oscillator 10 in FIG. 1 formed of inverters 12. A tunable current source 14 may be provided to adjust the delay within each inverter 12 to vary the oscillation frequency to some extent.
One feature inherent in ring oscillator designs is that multiple phases, obtained at tapped outputs 16-18 of inverters 12, are automatically generated at no additional expense. These multiple phases may be used, for example, for clocking a number of circuits within a system.
FIG. 2 shows a ring oscillator 20 using inverters 22 which have a differential input and output. When differential inverters 22 are used, then an odd or an even number of inverters 22 in the ring is possible since the ring phase shift can be created at any point in the series string, such as the crossover identified at numeral 24. Differential multiple phases are produced at the outputs of the various inverters 22.
Each of the inverters in FIGS. 1 and 2 are typically formed of CMOS transistors. The delay (switching speed) of each inverter is undesirably very dependent upon the control input voltage. Thus the gain constant (switching speed versus control input voltage change) tends to be relatively high and very dependent upon process parameters such as MOS conductance, threshold voltage, and resistor sheet rho (if used in support biasing). This high gain constant can exacerbate voltage controlled oscillator (VCO) phase noise when the ring oscillator is used in phase-locked loop applications. External influences such as supply voltage variations, temperature gradients, and substrate noise coupling also contribute to the overall phase noise of these oscillators.
What is needed is a ring oscillator for providing multiple phases having improved frequency stability.